Investors cheered last week's news that Chevron and its partners had discovered potentially the largest U.S. oil find in a generation, a deposit of as much as 15 billion barrels deep beneath the Gulf of Mexico.

At the Sierra Club in Washington, Athon Manuel, the environmental group's director of lands protection, could only shake his head in dismay.

"This isn't going to ease our dependence on foreign oil," he said. "U.S. oil consumption is like a big bucket. We're never going to fill that bucket with domestic production."

The danger, as he sees it, is that ordinary Americans will be lulled by Chevron's announcement into a renewed sense of complacency just as awareness is growing about the need for the United States to wean itself from its perilous oil addiction.

The nation consumes nearly 21 million barrels of oil every day -- a quarter of global supply. About 60 percent of U.S. oil demand is met by foreign providers, including our friends at OPEC.

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The situation is so severe, and so politically charged, that President Bush (a former oil man) has taken in recent months to calling for the United States to lessen its dependence on foreign oil. "Problem is, we get oil from some parts of the world and they simply don't like us," he said the other day.

True enough. A bigger problem, however, is that our thirst for oil continues to grow every year -- as does that of China, India and other major consumers -- while available supply becomes increasingly scarce.

The Energy Department is forecasting that world oil production will peak around 2037 or so. Some experts say we're already there. In any case, once that peak is hit, demand will steadily outstrip supply, and you don't have to be John D. Rockefeller to understand what that'll do to oil prices and virtually all economic sectors.

This is what makes the Chevron find so troubling from a conservation perspective. According to Cambridge Energy Research, the newly discovered oil fields could eventually produce up to 800,000 barrels per day -- a small fraction of the country's daily fix of 21 million barrels.

"If people don't do the simple arithmetic, they could think the new find is a lifetime supply of oil," said Pietro Nivola, an energy-industry expert at the Brookings Institution in Washington. "Many people might not realize that this is only a small addition to the supply side."

The Chevron find is significant, to be sure. It has the potential to boost current domestic reserves of 29 billion barrels by up to 50 percent (assuming there are 15 billion barrels of oil waiting down there; the total could actually be closer to 3 billion).

The find also highlights advances in deepwater drilling, raising the possibility that other large oil fields await discovery far off the coasts. The Gulf of Mexico deposit -- nicknamed "Jack" -- is under 7,000 feet of water and is more than 20,000 feet below the sea floor.

"Chevron continues to demonstrate its leading position employing deepwater exploration technology to develop new supplies of U.S. crude oil and natural gas with projects such as Jack," said George Kirkland, a company executive vice president.

"Our strong strategic position in the deepwater Gulf of Mexico will continue to be a platform for future growth for years to come," he said.

And that's fine. For Chevron.

But if our goal, as the president says, is to reduce our dependency on foreign oil, we must reduce our dependency on all oil. And that's why this new find, perversely, can be seen as a setback for the country's energy security.

"There's not enough oil there to deal with the fundamental equation we're dealing with," said Jim Presswood, the Natural Resources Defense Council's lead lobbyist on energy matters.

"As it stands, the United States has just 3 percent of the world's oil reserves," he observed. "Even if the amount of oil in this new find is on the high end, we'll still have only about 4.5 percent of global reserves.

"The answer," Presswood said, "is not to find domestic resources. The answer is to diversify away from oil."

That's why he and others in the conservation movement warn against Americans seeing this latest find as proof that our oil worries are unfounded.

This last objective is the easiest to achieve yet has proven the most elusive. Under the government's Corporate Average Fuel Economy program, passenger cars must meet an average of 27.5 miles per gallon -- a standard that's remained largely unchanged for more than 20 years.

According to the federal government's fueleconomy.gov Web site, the Toyota Prius averages nearly 50 mpg. Clearly it's not unreasonable to expect higher standards from other cars as well.

"We can't drill our way out of this problem," said the Sierra Club's Manuel. "There just isn't that much oil in the United States."